1. Field of the Invention
The present invention relates in general to communication systems and components, and related methods. More particularly, the present invention is directed to actively controlled antenna arrays and methods of actively controlling the elevation radiation pattern of an antenna array.
2. Description of the Prior Art and Related Background Information
Modern wireless antenna array implementations generally include a plurality of radiating elements that may be arranged over a reflector plane defining a radiated (and received) signal beamwidth and elevation plane angle (also known as tilt angle). The elevation plane angle antenna beamwidth can be advantageously modified by varying amplitude and phase of an RF signal applied to respective radiating elements. The elevation plane angle antenna beamwidth has been conventionally defined by Half Power Beam Width (HPBW) of the elevation plane angle of the beam relative to a bore sight of such antenna array. In such antenna array structure, the radiating element positioning is critical to the overall beamwidth control as such antenna systems rely on accuracy of the amplitude and phase angle of the RF signal supplied to each radiating element. This requires significant restrictions of the tolerance and the accuracy of a mechanical phase shifter to provide required signal division between various radiating elements over various elevation plane angle settings.
Real world applications such as terrestrial telephony often call for a high gain antenna array with beam down tilt and/or azimuth beamwidth controls that may incorporate a plurality of mechanical phase shifters to achieve such functionality. High gain, multi element antenna arrays are well known in the art (phased array antennas) and generally incorporate a linear phased array with equal or unequally spaced radiating elements. By controlling the phase and amplitude of excitation to each radiating element, the radiation beam direction and the shape of the beam radiated by the array can be dynamically controlled.
Such highly functional antenna arrays are typically retrofitted in place of simpler, lighter, and less functional antenna arrays. Hence, the weight and wind loading of the newly installed antenna array cannot increase significantly. Phase and amplitude accuracy of a mechanical phase shifter generally depend on its construction materials. Generally, highly accurate mechanical phase shifter implementations require substantial amounts of relatively expensive dielectric materials and rigid mechanical support. Such construction techniques result in an increased assembly volume, weight, and manufacturing costs. Additionally, mechanical phase shifter configurations that have been developed utilizing lower cost materials may fail to provide adequate passive intermodulation suppression under high power RF signal levels. Consequently, due to these design limitations, unwanted upper side lobe suppression at different down tilt angles may occur. Additional constraints related to upper side lobe suppression requires precise amplitude signal division for each radiating element used in such an antenna array which, in turn, may require variable amplitude signal division in addition to phase shift as provided by such phase shifter. Adding controlled amplitude variation to a high power phase shifter is problematic and further complicates its implementation.
Terrestrial telephony, such as cellular radio networks, extensively use antenna array antennas to attain enhanced geographical coverage which requires that the desired radiation pattern is known beforehand. A radiation pattern of a typical multi element radiation array may have a main lobe and numerous side-lobes. The center of the main lobe is defined as being equidistant between the two −3 dB roll off points is the antenna's direction for maximum gain.
Based on network coverage requirements, cellular radio network operators often have to adjust the vertical radiation pattern of the antennas, i.e. the pattern's cross-section in the vertical plane. When required, alteration of the vertical angle of the antenna's main beam, also known as the “tilt”, is used to adjust the coverage area of the antenna. Antenna angle of tilt have been implemented both mechanically and electrically, either individually or in combination utilizing remote control capabilities.
Antenna beam tilt angle may be adjusted mechanically by moving antenna elements relative to the vertical axis which is commonly referred to as a “mechanical tilt”. As described above, antenna beam tilt angle may be adjusted electrically by changing phase of signals fed to or received from each radiating element of the antenna array without moving antenna structure which is referred to as “electrical tilt”. When used in a cellular network, an antenna array vertical radiation pattern has to meet several key parameters. First, the antenna must provide high boresight gain over a useful beam width angle. Second, the antenna must exhibit first and second upper side lobe levels suppression. And finally, the antenna must suppress side lobe levels below a set limit over full range of beam down tilt angles.
The aforementioned requirements are mutually conflicting. For example, increasing the boresight gain may increase side lobes as well as side lobe angles and levels over various down tilt angles. It has been established that, if first and second upper side lobe levels are less than −15 dB, a workable compromise for the overall antenna performance can be achieved.
Generating a required angle of electrical tilt from a shared antenna has thus far resulted in compromises in the performance of the antenna. For example, the boresight gain decreases in proportion to the cosine of the angle of tilt due to a reduction in the effective aperture of the antenna. This effect is unavoidable and happens in all antenna designs. Further reductions in boresight gain may result as a consequence of the method used to change the angle of tilt.
In a conventional cellular network deployment Base Station (BS), an electrical tilt equipped antenna is coupled via a cable run to a suitable multi carrier transmitter. Typically, multi carrier transmitters may employ individual single carrier High Power amplifiers (PAs) for amplifying individual carrier signals produced by transceivers. RF outputs from the single carrier high power amplifiers are combined using high isolation cavity combiners, passed through receive—transmit duplexers before being coupled to a tower cable run (or RF wave guide) coupled to a tower mounted antenna. Such configuration is highly inefficient as individual RF amplifier outputs are attenuated due to losses associated with cavity combiners, duplexers, and the tower cable run connecting amplifier output to the an antenna.
An improved BS may employ multi carrier amplifiers which amplify individual RF carriers within a single amplifier. Such multi carrier power amplifiers (MCPA) utilize linearization schemes, which are well known in the art, to provide RF output that has reduced intermodulation distortion (IMD) and noise signal levels due to amplification nonlinearities present in the MCPA. However, tower cable run losses and duplexer losses are still present and must be accounted.
To further reduce insertion losses present in tower cable run and duplexers, the BS equipment must be mounted as close as possible to antenna. To achieve this, a Remote Radio Head (RRH) is mounted in the immediate proximity of the antenna. A RRH typically employs a linearized PA transmitter to provide RF carrier signals, while suppressing intermodulation and noise signal levels due to amplification produced within PA section of the RRH. Numerous linearization schemes known in the art can be employed in RRH transmitter PA section to provide appropriate IMD and noise level suppression. In all aforementioned operational deployments, the combined transmitter RF output from a common antenna port must have IMD and noise levels suppression as dictated by appropriate regulatory limits. In general, higher combined output levels require increased IMD and noise levels suppression since some of these limits have absolute power levels that can not be exceeded.
Placing a high power PA in close proximity to an antenna introduces a host of technical challenges related to PA linearity and efficiency as determined by PA's operating range on a characteristic Amplitude to Amplitude Modulation (AM-AM) and Amplitude Modulation to Phase Modulation (AM-PM) curves. Modern cellular systems employ complex, digitally modulated RF signals which tend to require highly linear PA operation. Maintaining desired output signal linearity while providing efficient operation is a highly desired characteristic for a PA. PA power efficiency can be calculated by dividing total power delivered to a load by the total power supplied to the amplifier. Depending on the bias class of the amplifier, output stage efficiency can be as low as 7-10 percent for class A amplifiers to as high as 45 percent for Doherty class amplifiers. Unfortunately, there is performance tradeoff between linearity and efficiency in PAs as highly linear operating class PAs (Class A for example) tend to be least power efficient when compared to similarly capable Class AB. Additionally, highly efficient amplifiers are required for tower mounted operation since conventional forced air cooling techniques add bulk and reduce reliability (as fans tend to fail when exposed to elements). In previous attempts, keeping a PA operating in high efficiency operation was found difficult to achieve due to dynamic nature of amplified signals, further being complicated by the antenna beam tilting.
R. C. Johnson, Antenna Engineers Handbook, 3rd Ed 1993, McGraw Hill, ISBN 0-07-032381-X, Ch 20, FIG. 20-2 teaches a well known method for adjusting a phased array antenna's electrical down tilt angle. A suitable radio frequency (RF) carrier signal is fed to input port of antenna array from a transmitter and divided among the antenna's radiating elements. Each radiating element is fed from a respective variable phase shifter so that signal phase can be precisely adjusted to vary the antenna array electrical down tilt. As noted previously, the division of power to antenna radiating elements must be controlled so as to provide satisfactory side lobe levels for a given boresight gain. It is highly desirable to maintain phase front for all downtilt angles so that the side lobe levels are not increased above set limits. However, this has been very difficult to achieve since practical phase shifters exhibit insertion loss variation over the range phase shift settings thus introducing RF signal division inaccuracies—contributing to the increased side lobe levels. Consequently, there is a need to provide a simpler method to adjust antenna down tilt beam, while providing enhanced upper side lobe suppression.